When two consecutive intervals are found with a slope greater than -2 P/lOO 

 ft or when one interval is found with a slope greater than /lOO ft, the 

 lower hound of the last slope with value less than -2^/l00 ft is recorded. 

 The slope between the two recorded hounds is computed and recorded as a 

 gradient. An identical search is begun at the last examined interval. When 

 all such gradients have been examined, the data for the gradient with the 

 greatest absolute value is recorded for output as the maximum gradient.* If 

 no gradient meets the requirement, all col\rans for the maximum gradient are 

 recorded as zero on the master card. Exceptions to this procedure are: 



a. Gradients less than Uo feet thick are not recorded unless the 

 slope is less than -25°F/100 ft. 



b. If a maximum gradient continues to the end of the trace and the 

 last interval meets the requirements for inclusion in the gradient, the 

 depth of that interval is recorded as the lower bound of the maximum gra- 

 dient. If the last interval has a negative slope but does not meet the 

 requirements of a maximum gradient and the next to the last interval is 

 included in the maximum gradient, the lower depth of the latter interval 

 is recorded as the lower bound of the maximum gradient. In either event, 

 coliomn 68 of the master card is X overpunched and the value of the maxi- 

 mum gradient is preceded by a minus sign on print out. 



h. To determine the mean gradient the digitized trace is searched for 

 the first interval below a recorded surface effect with slope less than 

 -I'^/lOO ft. The upper bound of this interval is recorded as the upper 

 bound of the mean gradient. A search is made below this point for the 

 lowest temperature of the digitized trace. The depth of the lowest 

 temperature is recorded as the lower bound of the mean gradient. The 

 slope between the upper and lower bovinds is computed and recorded as the 

 mean gradient. If positive gradients exist in the mean gradient, column 

 k9 of detail card no. 3 is X overpunched, and the mean gradient is pre- 

 ceded by a negative sign on print out.** If no gradient meets the requir- 

 ement, all columns for the mean gradient are recorded as zero. 



5. To determine the first gradient the portion of the digitized trace 

 below the recorded surface effect is searched for the first gradient with 

 slope less than -1°F/100 ft. The upper bound of this gradient is recorded 

 and the remainder of the trace is examined until either slopes in two 

 adjacent intervals equal to or greater than -1°f/100 ft are foiind or until 

 a slope greater than 0°/lOO ft is found. If the first gradient is not 

 found in the digitized BT trace, all columns for that entity are recorded 



*Some key punch and coding errors escaped a cursory check. The program 

 was written to reject any observation as being in error when the computa- 

 tion resulted in a gradient greater than 100°F/100 ft. Experience has 

 shown that this threshold value shoxold be reduced to 20°f/100 ft. 



**Experience shows that positive gradients are never detected by this 

 search. The entire BT trace should be searched for a positive gradient, 

 as well as the mean gradient. 



16 



